In general, spark electrodes of an ignition device for oil burners become coated with sprayed oil or combustion residue, so it is desirable that the spark electrodes are arranged at a position away from the sprayed fuel by a given distance and that the spark generated across the electrodes is transferred to the fuel by an air flow. As a result, the ignition device is required to generate a spark having a high power output, to reduce a loss induced by heat accumulated in circuit elements, and to effect a continuous operation for a long time period.
In order to satisfy such requirements, heretofore, it has been the common practice to provide an ignition device for internal combustion engines which is provided with a circuit constructed and arranged as shown in FIGS. 1(a) and 2(a). The ignition device shown in FIG. 1(a) is a prior art technique described in the U.S. Pat. No. 3,523,211. In FIG. 1(a), reference numeral 20 designates a direct current-direct current converter including a direct current source therein; a charging and discharging capacitor 4 connected in parallel with a series circuit including a silicon controlled rectifier 5 and a primary winding 12 of an ignition transformer 11 and also connected in parallel with a series circuit including a choke coil 13 and a diode 16; a secondary winding 14 of the ignition transformer 11; and 17 spark electrodes connected across the secondary winding 14.
As shown in FIG. 1(b), in such a prior art ignition device, if the silicon controlled rectifier 5 is caused to conduct at a time t=t.sub.o, the charge of the capacitor 4, which has been charged with polarities shown in FIG. 1(a), becomes discharged through the primary winding 12 and the silicon controlled rectifier 5 up to a time t=t.sub.1. The discharge continues even after the time t.sub.1 and begins to charge the capacitor 4 with polarities which are opposite to the polarities shown in FIG. 1(a) for a short time. During a short time between a time t.sub.o and this stage, a pulse of current flow in the primary winding 12 to generate a high voltage at the secondary winding 14 of the ignition transformer 11, thereby producing sparks across the spark electrodes 17. Such a conventional ignition device has the major drawback that after the time t.sub.1, the electromotive force which is opposed to the electromotive force induced in the primary winding 12 up to the time t.sub.1 is induced in the primary winding 12. As a result, a current i.sub.s flows again from the primary winding 12 through the silicon controlled rectifier 5, diode 16 and choke coil 13 to the primary winding 12. The current i.sub.s is a forwardly conductive current which functions to instantaneously eliminate some of the magnetic energy of the primary winding 12, thereby not only reducing the amount of charge to be stored in the capacitor 4 in the opposite direction, but also rendering the rise time of the voltage between the anode and cathode of the silicon controlled rectifier 5 too short so that the turn-off time of the silicon controlled rectifier 5 is not satisfied, thereby rendering the turn-off of the silicon controlled rectifier 5 difficult.
If the choke coil 13 is selected as having a large inductive value to avoid such a drawback, the small amount of charge accumulated in the capacitor 4 and having the opposite polarity could not be recovered within a short time and could not be utilized to generate spark or maintain an ionized condition, thereby lowering the oscillation frequency and rendering it impossible to obtain a spark having a high power output. In addition, the circuit becomes very expensive.
FIG. 2(a) shows another prior art technique which is described in the U.S. Pat. No. 3,665,908. Those circuit elements shown in FIG. 2(a) which are the same as those shown in FIG. 1(a) are designated by the same reference numerals.
In the prior art circuit shown in FIG. 2(a), an ignition transformer 11 is provided with an auxiliary tertiary winding 13 magnetically inductively coupled with a primary winding 12 and a secondary winding 14. The tertiary winding 13 is connected in series with a circuit including a diode 16 and a choke coil 12. The ignition transformer 11 is provided with an intermediate tap 22 which is connected to an anode of a silicon controlled rectifier 5 whose cathode is connected to a negative terminal of a charging and discharging capacitor 4. The silicon controlled rectifier 5 is connected in parallel with a series circuit including the tertiary winding 13, diode 16 and the choke coil 21.
The ignition device shown in FIG. 2(a) will operate as follows. As seen from FIG. 2(b) showing operative wave forms of various circuit elements, if the silicon controlled rectifier 5 is caused to conduct, the charge accumulated in the capacitor 4 is discharged through the primary winding 12 and silicon controlled rectifier 5 into a negative terminal of the capacitor 4. Contrary to the ignition device shown in FIG. 1(a), the ignition device shown in FIG. 2(a) is constructed such that the primary winding 12 and the tertiary winding 13 are wound in polarities shown by large dots 23a and 23b, so that the voltages induced in the primary winding 12 and tertiary winding 13 are directed in the same direction. As a result, the forwardly conductive voltage at the silicon controlled rectifier 5 and the diode 16 produced in the tertiary winding 13 within a period between a time t.sub.o and a time t.sub.1 causes the shortcircuit current i.sub.s to flow. But, the shortcircuit current i.sub.s does not flow within a time between t.sub.1 and t.sub.2. Thus, the charge on the capacitor 4 becomes reversed and the capacitor is charged in the opposite direction at the time t.sub.2. After the time t.sub.2, the capacitor 4 tends to return to its original condition, and as a result, a current is caused to flow through the silicon controlled rectifier 5 in the opposite direction at a time t.sub.3 and at the same time flows through the choke coil 21, thereby applying a voltage to the silicon controlled rectifier 5 in an opposite direction. The ignition device, therefore, has the advantage that it is possible to turn off the silicon controlled rectifier 5 in an easy manner. The ignition device, however, has the following drawbacks. In the first place, the shortcircuit current i.sub.s becomes very large, so that a current whose value is larger than that desired for the silicon controlled rectifier 5 instantaneously flows therethrough, and there is a risk of the silicon controlled rectifier 5 being broken down and it accumulates excessive heat. Secondly, at the time t.sub.3 a large current and high voltage are forced through and applied to the silicon controlled rectifier 5 causing a premature turnoff of the silicon controlled rectifier 5, thereby accumulating heat therein. Finally, in order to obviate such drawbacks, if the choke coil 21 is selected having a large inductance, as in the case of the ignition device shown in FIG. 1(a), the oscillation frequency becomes lower and at the same time the spark power output is decreased and the ignition device becomes expensive.